A UPS (Uninterruptible Power System) is an alternating current power supply with an energy storage device, which provides uninterruptible power supply to a load using the energy storage device such as a battery during power failures. When the mains power supply is available, the UPS regulates the mains power supply and supplies it to the load. When the mains power supply is not available (power failure), the UPS supplies alternating currents to the load using the energy storage device, so as to keep the load working normally and prevent the software and hardware of the load from damage.
FIG. 1 shows a basic circuit configuration of the UPS, which comprises an input end Tin receiving an AC power from the mains power supply and an output end Tout supplying the AC power to the load. There are an electro-magnetic interference filter EMI, switches SW1, SW2, SW3 and SW4, and a low frequency transformer TX between the input end Tin and the output end Tout. In the mains power supply mode, SW3 and SW4 are closed, and the AC power is delivered from the input end Tin to the output end Tout. At the same time, the AC power is delivered from a secondary side of the low frequency transformer TX to a circuit module AD for realizing AC/DC conversion. The circuit module AD rectifies the AC power into a DC power and then charges a battery B. When the mains power supply is not available and the UPS works in the battery mode, the circuit module AD works in inverter mode, and converts the DC power provided by the battery B into the AC power and provides the AC power to the secondary side of the transformer TX. A primary side of the TX provides the AC power to the output end Tout of the UPS circuit. The switches SW1 and SW2 are used to regulate the mains power supply voltage when it is too high or too low.
However, the efficiency requirements to the UPS have become increasingly higher with the proposition of environmental protection strengthening, in particular EPA Energy star. This type of UPS circuit has a big energy loss, which makes it difficult to meet the Energy star standard.
In the mains power supply mode, the UPS has a big self-loss even in a fully charged state. The loss mainly comprises: 1) an overall circuit conduction energy loss and a mains power supply components loss; 2) a loss for keeping the whole control system and the switching components operating; 3) an excitation loss of the low frequency transformer. Wherein, the excitation loss accounts for over 60% of the total no-load loss, and thus is the main cause of the no-load loss.
Core materials with lower magnetic losses may be used to reduce the excitation loss of the low frequency transformer. However, the cost is higher and the effect is limited. Another method to reduce the excitation loss is to disconnect the low frequency transformer from the mains power supply circuit in the mains power supply mode. In theory, in the mains power supply mode, if the battery B has been fully charged, the low frequency transformer does not need to power the battery B and disconnecting the low frequency transformer will not affect the UPS circuit. In practice, however, the battery has a self-discharge effect and the electrical energy stored therein will be drained slowly even if the battery does not power the load. In addition, some control circuits need to get a small amount of electrical energy from the battery. Consequently, the low frequency transformer is needed to charge the battery continuously. The method of disconnecting the low frequency transformer can completely eliminate the excitation loss of the low frequency transformer, but it is needed to get some energy from the mains power supply in consideration of the control circuits and the self-discharge of the battery. Therefore, an additional independent power supply is required to provide the energy, which causes a higher production cost.